1. Field of the Invention
The present invention relates generally to an active suspension system for an automotive vehicle. More specifically, the invention relative to derivation of vertical stroke speed as control parameter for active suspension control.
2. Description of the Background Art
U.S. Pat. No. 4,702,490, issued on Oct. 27, 1987 which has been assigned to the common owner to the present invention, discloses one of typical construction of an actively controlled suspension system, in which a hydraulic cylinder defining a working chamber is disposed between a vehicular body and a suspension member rotatably supporting a vehicular wheel. The working chamber of the hydraulic cylinder is communicated with a hydraulic circuit including a pressurized working fluid source. A pressure control valve, such as an proportioning valve assembly, is disposed in the hydraulic circuit, which is connected to an electric or electronic control circuit to be controlled the valve position. The pressure control valve is controlled the valve position by a suspension control signal produced in the control circuit for adjusting pressure in the working chamber and whereby controlling suspension characteristics.
On the other hand, European Patents 0 283 004, 0 285 153 and 0 284 053 discloses technologies for controlling the suspension systems constructed as set forth above, depending upon the vehicle driving condition for suppressing rolling and/or pitching of the vehicular body. Furthermore, Japanese Patent First Publication 62-289420, Japanese Patent First Publication 62-295714 disclose similar active suspension systems.
For example, Japanese Patent First Publication 62-289420 discloses suppression of vertical displacement of vehicular body by adjusting fluid pressure in a hydraulic cylinder to generate a force resisting against the vehicular body motion. For this, it becomes necessary to derive a vertical stroke or motion speed of the vehicular body. In the above-identified Publication, the vertical stroke speed is arithmetically derived on the basis of a vertical acceleration monitored by means of a vertical acceleration sensor. A vertical acceleration indicative signal produced by the vertical acceleration sensor is supplied to a low-pass filter which serves as an integrator for integrating the vertical acceleration and thus deriving the vertical stroke speed. The vertical acceleration indicative signal is also supplied to a band-pass filter for removing direct current component from the vertical acceleration indicative signal. Such filtering process is performed by digital filter for obtaining sufficiently high process speed.
In such prior proposed active suspension system, a drawback is encountered in derivation of the vertical stroke speed. Namely, the vertical acceleration indicative signal output from the vertical acceleration sensor may contain offset due to a temperature dependent drift. Such offset may serves to provide equivalent effect to supply stepwise input for the filter to cause swift variation of the vertical stroke speed upon starting-up of operation of the system. Such swift variation may cause activation of the active suspension system to initiate vibration suppressive operation despite of the fact that the vehicle body is maintained at stable state. This can lead vertical displacement of the vehicular body upon starting of system operation, i.e. turning ON of an ignition switch, to surprise the passengers. This clearly degrade the riding comfort of the vehicle.